This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2001-113693 filed on Apr. 12, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a screen printing apparatus for printing paste, such as cream solder or conductive paste, onto a substrate, and a method of screen printing.
2. Description of Related Art
In an electronic-parts-mounting process, screen printing is employed for printing paste, such as cream solder or conductive paste, onto a substrate. In the screen printing, a mask plate having pattern apertures corresponding to the places to be printed is placed over a substrate, and paste is printed onto the substrate through the pattern apertures by using a squeegee method.
In a conventional squeegee method and apparatus, such as disclosed in U.S. Pat. No. 5,996,487, Laid Open Japanese Patent H-8-323956 and Laid Open Japanese Patent 2000-255029, a lower edge of a plate-shaped squeegee slides in contact with a surface of the mask plate. The squeegee slides while being slanted relative to its squeegee-moving direction to form a predetermined attack angle between the squeegee and the surface of the mask plate. A squeegee head has a pair of squeegees whose squeegee-moving directions are opposite each other. Each squeegee is individually operated in turn. Accordingly, the squeegees alternately carry out the squeegeeing operation by reciprocating the squeegee head.
The attack angle is required to be constant during the squeegeeing operation in order to maintain a stable printing condition for the screen printing. However, if the surface of the substrate is warped or has undulations, the attack angle changes according to the degree of the warp of the surface of the substrate. In that case, the printing condition becomes unstable and the amount of the cream solder supplied from the squeegee head fluctuates.
Accordingly, it is an object of the present invention to provide a screen printing apparatus and method capable of stabilizing the printing condition and improving the printing quality.
According to a first aspect of the present invention, a screen printing apparatus has a squeegee head for printing paste onto a substrate, a horizontal driver for moving the squeegee head in a horizontal direction on a mask plate, a vertical driver for moving the squeegee head in a vertical direction with respect to the mask plate, and a presser for generating pressure to push the squeegee head against the mask plate. The squeegee head includes a pair of squeegees having opposite squeegeeing directions, each of the squeegees having a lower end for sliding in contact with the mask plate with a predetermined attack angle with respect to the squeegeeing directions, respectively, a retainer for connecting the squeegees and for retaining the squeegees so that they are rotatable with respect to a horizontal axis that is at a right angle to the squeegeeing directions, and a paste cell for storing the paste between the squeegees and the mask plate when the squeegees are in contact with the mask plate. Accordingly, the attack angle is kept constant to improve the printing quality even if a substrate has a curved portion or is in a slanted condition.
According to another aspect of the present invention, a method of screen printing for printing paste onto a substrate via a pattern aperture of a mask plate by sliding a pair of squeegees on a mask plate, includes the steps of placing a first substrate under the mask plate, lowering the squeegees to make contact with the mask plate, holding the squeegees so that they are movable on the mask plate such that an attack angle formed between the squeegees and the mask plate is an acute angle while maintaining the contact between the squeegees and the mask plate, supplying paste to a paste cell formed between the squeegees and the mask plate when the squeegees are in contact with the mask plate, sliding the squeegees on the mask plate in a first direction to complete the printing of the paste onto the first substrate while holding the squeegees so that they are movable on the mask plate and such that the attack angle formed between the squeegees and the mask plate is kept constant, replacing the first substrate with a second substrate, and sliding the squeegees on the mask plate in a second direction opposite to the first direction to complete the printing of the paste onto the second substrate while holding the squeegees so that they are movable on the mask plate and such that the attack angle formed between the squeegees and the mask plate is kept constant. Accordingly, the attack angle is kept constant to improve the printing quality even if a substrate has a curved portion or is in a slanted condition.
According to another aspect of the present invention, a method of screen printing for printing solder onto a substrate via a pattern aperture of a mask plate by sliding a pair of squeegees on the mask plate, includes the steps of placing a first substrate under the mask plate, lowering the squeegees to make contact with the mask plate such that an acute angle is formed between the squeegees and the mask plate with respect to a squeegeeing direction of each squeegee, supplying the solder to a solder cell formed between the squeegees and the mask plate when the squeegees are in contact with the mask plate, sliding the squeegees on the mask plate in a first direction to complete the printing of the solder onto the first substrate, replacing the first substrate with a second substrate; and sliding the squeegees on the mask plate in a second direction opposite to the first direction to complete the printing of the solder onto the second substrate. Accordingly, it is not necessary to lift and lower each squeegee for switching the squeegee when the squeegeeing direction is reversed. Therefore, loss of time caused by lifting and lowering each squeegee is prevented, and the efficiency of the screen printing is improved.